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But chance remains to mitigate severe consequences

By Cassa Niedringhaus

Staff Writer

Posted:
08/12/2018 02:00:00 PM MDT

Keith Musselman, a University of Colorado research associate and snow hydrologist, ski tours at Mount Robertson in the Canadian Rockies, where he was inspired to study the links between a warming climate and increased rain-on-snow flooding. (Keith Musselman Courtesy Image / Daily Camera)

In a warmer climate around the end of the century, rain-on-snow flooding could more than double and increase the risks for people living in certain sections of North America.

The risk would increase most in mountainous regions, namely the Sierra Nevada range in California, the Colorado River headwaters and the Canadian Rocky Mountains, according to a study published this week by researchers at the University of Colorado and the National Center for Atmospheric Research.

As rain falls, the storm's moist air condenses on the snow pack and causes it to melt. The moisture from the rain and melted snow combine to create a deluge that poses risks downhill. Such floods could become more frequent and more intense by the end of the century because of human-caused climate change — but they're also difficult to predict.

"We need to be able to anticipate how that would impact life and property," said Keith Musselman, a research associate at CU's Institute of Arctic and Alpine Research and lead author of the study, which was published Aug. 7 in the Nature Climate Change journal.

Musselman was first inspired to study rain-on-snow flooding while he was living as a postdoc outside Banff in the Canadian Rockies. In June 2013 — just three months before a similarly sized rainstorm in Colorado — a storm dropped rain on late-lying snow in the Canadian Rockies, and the resulting flood became one of the most expensive natural disasters in the country's history.

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"Our town was cut off from the east and the west," Musselman said. "We couldn't leave our little valley town, and a lot of people lost their houses as a result of the flood. As a hydrologist, and also a trained civil engineer, I recognized the challenges associated with forecasting such an event, understanding what was happening to create such a large magnitude flood."

This research, he said, can contribute to our understanding of such storms and what dangers they'll pose as the climate warms throughout this century. Warming would change the timing and intensity of floods, like the Colorado Front Range floods of 2013.

"What happens if the (Colorado) flood of September 2013 came in December?" Musselman said. "There's no reason to think it wouldn't be possible. The challenge is anticipating the possibility."

The study

The researchers used a massive data set NCAR developed to compare current weather simulations with weather simulations for the end of the century.

They found there would be a shift in the frequency of rain-on-snow storms away from the Pacific Northwest, an area that currently has them more often, because the region would have a smaller snow pack. They found that the storms would shift into colder regions and regions at higher elevations, areas that don't currently have frequent rain-on-snow events.

In a few areas — the Sierra Nevada, Colorado River headwaters and Canadian Rockies — they found an overlap where there has historically been floods and where the floods would increase in the future.

"This overlap was where, historically, there were a lot of very destructive rain-on-snow events, and in the future there will be even more and more intense rain-on-snow events," Musselman said.

These storms and floods would increase the pressure on water management and reservoirs and strain existing infrastructure, Mussleman said. As the timing and magnitude of floods changed, reservoir managers would have to adjust their timelines for when they release water.

"It really comes down to reservoir management," Musselman said. "We use reservoirs in the west to store water — it's such an important resource for us — but we also use reservoirs in many cases to prevent floods, and that's where the challenge lies."

In Colorado specifically, he said, it's more difficult to know exactly how increasing rain-on-snow would affect the state. It's not common now, so there isn't as much historical information to use as a comparison.

However, he said, it's likely the state would see more frequent winter rains at higher elevations. Rain-on-snow would affect the ski industry and fisheries, for example.

"Here on the Front Range, we're acutely aware of the impacts of floods," Musselman said. "These types of studies really highlight the potential impacts of a changing climate and the importance of considering appropriate action at all levels of state and government."

The only silver lining of the study is that it assumes that greenhouse gas emissions continue as they are now, which means that curbing emissions could reduce the risks of the rain-on-snow floods that the researchers have predicted.

"This is a business-as-usual emissions scenario," Musselman said. "It's under the premise that greenhouse gas emissions won't be curbed globally by the end of the century, and they'll continue at the current rate.

"The silver lining, as they say, is that there's an opportunity to mitigate these estimates."

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